基类指针可以指向一个派生类对象。为什么反之亦然不是真的？ [英] A Base Class pointer can point to a derived class object. Why is the vice-versa not true?
问题描述
基类指针可以指向派生类对象。为什么反之亦然不是真的没有铸造？
逻辑上，一个基类没有足够的派生类的信息，但派生类也应该有基类的信息。
我在这里缺少一些基础知识。
A Base Class pointer can point to a derived class object. Why is the vice-versa not true without casting? Logically a base class would not have enough information of the derived class but a derived class should have the information of the base class as well. I am missing some basics here.
推荐答案
如果我告诉你我有一只狗，我有一只狗。
If I tell you I have a dog, you can safely assume that I have a dog.
如果我告诉你我有一只宠物，你不知道那只动物是一只狗，它可能是一只猫，甚至可能是一只长颈鹿。不知道一些额外的信息，你不能安全地假设我有一条狗。
If I tell you I have a pet, you don't know if that animal is a dog, it could be a cat or maybe even a giraffe. Without knowing some extra information you can't safely assume I have a dog.
同样，派生对象是一个基类对象（因为它是一个子类）可以通过基类指针指向。但是，一个基类对象不是一个派生类对象，所以它不能被赋给一个派生类指针。
similarly a derived object is a base class object (as it's a sub class), so it can be pointed to by a base class pointer. However, a base class object is not a derived class object so it can't be assigned to a derived class pointer.
（你现在听到的吱吱声是类比拉伸）
(The creaking you will now hear is the analogy stretching)
假设你现在想为我的宠物给我买礼物。
Suppose you now want to buy me a gift for my pet.
在第一种情况下，你知道它是一只狗，你可以给我一个皮带，每个人都很高兴。
In the first scenario you know it is a dog, you can buy me a leash, everyone is happy.
在第二种情况下，我没有告诉你我的宠物是什么，所以如果你要买我一个礼物，无论如何你需要知道的信息，我没有告诉你（或只是猜测），你买我一条皮带，如果事实证明我真的有一只狗，每个人都快乐。
In the second scenario I haven't told you what my pet is so if you are going to buy me a gift anyway you need to know information I haven't told you (or just guess), you buy me a leash, if it turns out I really did have a dog everyone is happy.
然而，如果我实际上有一只猫，那么我们现在知道你做了一个坏的假设（演员），并在一个皮带上有一个不快乐的猫（运行时错误）。
However if I actually had a cat then we now know you made a bad assumption (cast) and have an unhappy cat on a leash (runtime error).
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